Tin-lead solders are commonly employed to provide interconnections in electronic circuitry such as between the terminals of integrated circuits and wiring pads provided upon printed wiring boards, to name just one typical application.
Electronic machines used to interconnect such electronic components and circuits to one another employ a variety of processes relating to electronic soldering, which typically utilizes solder in the molten state.
Substantially all of such processes utilize copper surfaces initially wetted by the solder to form an intermediary structure in order to produce a good electrical connection between the surfaces being joined.
One of the inherent elements of soldering is the employment of a suitable molten solder. Molten solder is corrosive and thus causes a small amount of copper from the wetted surfaces to be taken up in solution in the molten (liquid) solder. Copper in solution is tolerable up to a concentration of 0.3 percent by weight at typical soldering temperatures. Above this level, copper becomes saturated and precipitates as a tin-copper intermetallic compound, which is a crystalline structure having a needle-like configuration which produces rough and gritty solder.
The solubility of copper in solder is a function of temperature and solder composition. As temperature increases, copper solubility increases. As tin content increases, the solubility also increases. The solder of principle interest for electronic soldering is 63/37 (the eutectic alloy), which is comprised of sixty-three percent by weight of tin and thirty-seven percent by weight of lead. The following discussion will refer to the 63/37 alloy.
The limit of solubility of copper in solder is referred to as the saturation point which is expressed as percent by weight of copper in the molten solder bath at a specific temperature. A solder bath is said to be in equilibrium when the percent of copper in the bath is at the saturation point. The formation of a tin-copper intermetallic compound begins at the saturation point and progresses as the temperature of the molten bath is reduced. If a particular molten solder composition is in equilibrium at a particular temperature, then excess crystals will be apparent as floating solids and the liquid solder will be in equilibrium.
It is thus advantageous to remove copper from the molten bath and/or to reduce its percent weight in the molten bath to significantly reduce and/or eliminate the presence of rough and gritty solder due to the presence of an undesirable excess of copper.